Heat 1 tablespoon of the oil in a large skillet over medium heat. Add the chopped onion and cook, stirring occasionally, until beginning to soften, 2 to 3 minutes. Add the beef, cayenne, ½ teaspoon salt, and ¼ teaspoon black pepper. Cook, breaking up the beef with a spoon, until no longer pink, 3 to 5 minutes more.
Heat broiler. Brush one side of each tortilla with 1 teaspoon of the remaining oil. Place 2 tortillas, oiled-side down, on a foil-lined baking sheet. Top with the beef mixture, Monterey Jack, and remaining tortillas (oiled-side up). Broil until crisp and the cheese melts, 1 to 2 minutes per side.
Meanwhile, in a large bowl, toss the watercress, corn, and sliced onion with the lime juice, the remaining tablespoon of oil, and ¼ teaspoon each salt and black pepper. Cut the quesadillas into wedges and serve with the salad.